Steel cords currently used in tyres for use with trucks or buses include a two-layer steel cord which comprises an inner layer comprising of a plurality of twisted filaments, and an outer layer comprising of a plurality of twisted filaments disposed around the first layer while being integral with the inner layer. A further example of steel cord used in tyres for trucks or buses includes a three-layer steel cord which further comprises an outermost layer consisting of a plurality of twisted filaments disposed around the outer layer of the two-layer steel cord.
A representative standard for a three-layer steel cord is a “3+9+15” construction. In brief explanation of a method for producing the three-layer steel cord having such a construction, three filaments are first twisted in a certain direction to form an inner layer. Subsequently, nine filaments are twisted in the same twist direction as that of the inner layer while using a twist pitch length different from that of the inner layer to form an intermediate layer, with the intermediate layer surrounding the inner layer while being integral with the inner layer. After that, fifteen filaments are twisted in a twist direction opposite to that of the intermediate layer while using a twist pitch length different from that of the intermediate layer to form an outer layer, with the outer layer surrounding the intermediate layer while being integral with the intermediate layer. A further filament may be twisted in a direction opposite to that of the outer layer while using a desired twist pitch length to form a spiral wrap, with the spiral wrap surrounding the outer layer while being integral with the outermost layer.
This three-layer steel cord is high in strength by virtue of the plurality of filaments, so that it is generally used for a carcass-reinforcing steel cord or a belt-reinforcing steel cord in large tyres used for trucks or buses. However, in the three-layer steel cord, the inner layer, intermediate layer, outer layer and spiral wrap are twisted in such a manner that these layers have different twist pitch length and/or different twist direction between the adjacent layers. Consequently, in the manufacture of such a steel cord, a twisting process is carried out for the inner layer, intermediate layer, outer layer, and spiral wrap, individually. This results in a reduction of productivity and an increase in the manufacturing costs.
EP 1067236 suggests to solve this problem by providing a two-layer steel cord including an inner layer consisting of three filaments and an outer layer consisting of seven or eight filaments and surrounding the inner layer, in which the inner layer and the outer layer are twisted in such a manner that they are equal in a twist direction and a twist pitch length to each other. According to EP 1067236, such steel cord has also an improved penetration of rubber into the steel cord, such that the steel cord has an improved durability upon application to tyres.
A similar cord is also disclosed in U.S. Pat. No. 5,473,878. In detail, one of the embodiments disclosed in U.S. Pat. No. 5,473,878 is a metallic cord for the reinforcement of elastomer used in the manufacture of tyres, which comprises a core composed of three metallic core filaments twisted about each other and a coaxial layer of eight metallic layer filaments arranged around the core filaments. The metallic core and layer filaments are twisted in the same direction at the same pitch. The metallic core filaments have a first diameter and the metallic layer filaments of the coaxial layer have a second diameter equal to the first diameter. According to U.S. Pat. No. 5,473,878, the result of constructing a cord with the size relationships afforded by the selection of the core and layer filament sizes and the number of layer filaments, is three spacings between the layer filaments which allows for direct contact of the core with the calender rubber conventionally applied to the cord. That is, a large surface area of the layer filaments is exposed, while at the same time exposing the core filaments to significant rubber penetration. Moreover, according to U.S. Pat. No. 5,473,878, the rubber penetration into the spacing due to the relationship of the sizes of the core filaments and the layer filaments provide effective interlocking between all of the filaments forming the cord.
U.S. Pat. No. 5,321,941 discloses a steel cord for the reinforcement of rubber articles. The steel cord comprises a core, consisting of two to four steel filaments and a layer of steel filaments completely enclosing said core. All the filaments have a diameter between 0.15 and 0.40 mm and are twisted in the same direction and at the same pitch. The cord has substantially over its entire length cross-sections which present gaps between adjacent filaments of the layer. The accumulated gaps are at least 0.03 mm. At least one filament of the layer has been preformed substantially differently from the other filaments. According to U.S. Pat. No. 5,321,941, appropriate preforming of at least one filament of the layer “creates”—together with the possible difference in diameters between the core filaments and the layer filaments—the gaps in the layer and, consequently, promotes rubber penetration. Moreover, according to U.S. Pat. No. 5,321,941 this cord also reduces core migration. One embodiment disclosed in U.S. Pat. No. 5,321,941 is a steel cord comprising three core filaments surrounded by nine layer filaments, three of which are preformed.
In order to improve rubber penetration without increasing the cord diameter, US 2004/0060632 suggests to provide metallic cords being made up of six to twelve metallic filaments whose diameter (d) is in a range of from 0.15 to 0.45 mm. The six to twelve metallic filaments include waved filaments and unwaved filaments, each of the waved filaments being two-dimensionally waved at a wave pitch Pw and wave height (h), the wave pitch Pw being in a range of from 5.0 to 35.0 times the diameter (d) of the filament, and the wave height (h) being in a range of from 0.2 to 4.0 times the diameter (d) of the filament. The metallic filaments are twisted together into the cord at a twist pitch Pc of from 10 to 40 mm so that the two-dimensionally waved filaments are each subjected to a certain rotation around its axial. According to US 2004/0060632, until the cord is rubberized, the two-dimensionally waved filaments become unstable, and gaps which effectively work on rubber penetration can be formed between the filaments without increasing the thickness of the finished cord. Two embodiments disclosed in US 2004/0060632 are examples of bead reinforcing cord composed of nine and twelve filaments, respectively. In a first example, five waved filaments and four unwaved filaments as a bunch are twisted into the cord while interchanging the positions of two filaments. In a second example, six waved filaments and six unwaved filaments as a bunch are twisted into the cord while interchanging the positions of two filaments. According to US 2004/0060632, when a plurality of filaments, as a single bunch, are twisted, the relative positions of the filaments are substantially not changed along the longitudinal direction, and closed spaces are liable to be formed among the filaments. Such closed spaces can be broken by upsetting the positional balance of the filaments. In order to effectively upset the positional balance, the positions of two filaments are intentionally interchanged.
WO 01/10656 relates to a steel cord, for reinforcing rubber articles, of the “3+6” type, specifically three core wires and six crown wires, having the same diameter of between 0.30 mm and 0.40 mm, in particular equal to 0.35 mm. According to WO 01/10656 this cord allows good penetration of the rubber from the outside towards the core wires, offering good resistance to the stresses in working conditions.